This invention relates to semiconductor lasers and more particularly, to an electrically pumped edge-emitting semiconductor laser based on one- or two-dimensional photonic bandgap structures.
With the growth of photonic technologies and integrated optics, semiconductor lasers have seen increasing use in optical telecommunications and other optoelectronic applications. Optical communications can require high-speed modulation of a light signal with low-loss transport over long distances, temporal coherence, and wavelength multiplexing. Furthermore, the ability to integrate multiple functions on the same monolithic device or chip may offer efficiency, reliability, and cost improvements over current discrete device functions for such applications. Current semiconductor laser sources have several shortcomings for use in such photonic integrated circuits.
Photonic crystal microcavities have been recently demonstrated that may enable low threshold lasers that operate at lasing wavelengths important for optical communications. Photonic crystals comprise periodic dielectric structures that have photonic bandgaps (PBGs) that prevent light with energy within the PBG from propagating in certain directions. An active medium can be introduced within the photonic crystal structure having a lasing frequency that lies within the PBG. The photonic crystal structure can thereby provide reflecting walls, forming a resonant optical cavity. The reflectivity of PBG mirrors can be controlled by engineering of the photonic crystal structure. Furthermore, if the spacing between the PBG reflectors is made very small, thereby forming a microcavity of the active medium, it is possible to increase the mode spacing of the microcavity such that only one mode falls within the emission linewidth of the active medium. Additionally, these PBG semiconductor lasers can have high cavity quality factors, Q, to reduce the emission linewidth and improve temporal coherence of the emitted light, thereby enabling many multiplexed signals to be carried on a single fiber for communications applications.
Painter et al. have described a laser microcavity formed from a single defect in a two-dimensional (2D) photonic crystal. O. Painter et al., xe2x80x9cTwo-Dimensional Photonic Band-Gap Defect Mode Laser,xe2x80x9d Science 284, 1819 (1999). Their optical microcavity consisted of a half wavelength-thick waveguide for transverse (vertical) confinement and a 2D photonic crystal mirror for lateral localization of the guided mode. The laser had an air-bridge geometry, wherein the waveguide slab was suspended in air so that air was the low-index cladding material. The microcavity laser was fabricated in indium gallium arsenic phosphide (InGaAsP) with optical gain provided by strained quantum wells with a peak emission wavelength of 1.55 xcexcm. Pulsed lasing action was achieved by optical pumping with a semiconductor laser focussed on the defect microcavity. However, a high incident laser pump power of 6.75 mW was required, because of the relatively low quality factor (Q of about 250) of the defect mode, poor cooling of the waveguiding slab, and inefficient optical pumping.
Zhou et al. have reported a 2D-PBG defect mode surface-emitting laser with electrical current injection into an InGaAs p-n junction with lasing at 0.9 xcexcm wavelength. Zhou et al. xe2x80x9cElectrically injected single-defect photonic bandgap surface-emitting laser at room temperature,xe2x80x9d Electronics Letters 36(18), 1541 (2000). The defect mode had a high quality factor of about 1164, due to good lateral confinement by the 2D-PBG structure and a bottom distributed Bragg reflector mirror. The laser had a relatively low threshold current of 300 xcexcA, due to efficient electrical pumping.
However, the surface-emitting semiconductor lasers of Painter et al. and Zhou et al. cannot be easily integrated with other optical components to provide a photonic integrated circuit. It is therefore an object of the present invention to provide a low-threshold, edge-emitting PBG semiconductor laser that can be efficiently pumped electrically, emits long-wavelength light, and can be easily integrated with other active and passive optical components. Stimulated emission can be achieved at a low threshold current in the semiconductor laser of the present invention, due to efficient carrier injection into the active region, good optical confinement, and efficient extraction of the emitted light. With a microcavity semiconductor laser of the present invention, cavity quality factors in excess of 1000 can be realized, enabling low threshold currents of tens of microamperes.
The present invention relates to an electrically pumped edge-emitting photonic bandgap semiconductor laser, comprising an active layer of semiconductor material defining a plane having an axial direction, comprising a periodic dielectric structure adapted to have a photonic bandgap, the periodic dielectric structure having an active region therein where the semiconductor material forms an active p-n junction having light emission at a lasing wavelength within the photonic bandgap; at least two cladding layers enclosing the active layer for optical confinement of the light emission to the axial direction; and a positive electrode connected to the p-type material and a negative electrode connected to the n-type material for electrical pumping of the active region to achieve light emission from the p-n junction in the axial direction of the semiconductor laser. The active region can further comprise an active p-i-n junction having a quantum well structure. The quantum well structure can be relaxed outside of the active region to enable efficient extraction of the emitted light. The at least two cladding layers can be electrically insulating cladding layers forming a current-carrying aperture therein having an axial dimension of less than the lasing wavelength within the semiconductor material to provide efficient carrier injection into the active region.